I. Field of the Invention
Superior nuclear shields may be constructed from gadolinium filled chloro-fluoro substituted ethylene polymers, particularly filled ethylene-chlorotrifluoroethylene or filled polychlorotrifluoroethylene. The fillers used in conjunction with these polymers comprise gadolinium compounds, preferably compounds such as gadolinium boride, gadolinium oxide, gadolinium aluminate and gadolinium aluminum borate.
II. Brief Description of the Prior Art
Nuclear shields which are intended to absorb neutrons are usually made from boron compounds such as elemental boron, boron oxide, boron carbide, etc. These compounds are usually refractory in nature, brittle and difficult or expensive to produce in precise or odd shapes. Recent attempts to use these boron compounds as fillers in plastics or glass have met with mixed results. In the case of glass artifacts, particularly borosilicate glass Raschig rings, the fragile nature of the glass as well as difficulty in processing tends to reduce any inherent advantages. The use of fluorine substituted polymers has been proposed since plastics are easier to process than glass; however, some of these fluorine substituted compounds, particularly polytetrafluoroethylene (PTFE), cannot be melt processed using conventional techniques and instead require special molding and extrusion methods consisting of formulating the granular plastic powder into the desired part under pressure and then sintering below the melt temperature to coalesce the particles. Moreover, the resulting fluorine-substituted polymeric compositions exhibit poorer properties when filled and degrade when exposed to radiation. Additionally, for many applications where the artifacts are to be exposed to high radiation dosages, it is necessary to employ a large amount, i.e., greater than 10% filler in the plastic materials. Such materials containing these large amounts of boron are extremely difficult to extrude and/or mold and cause significant wear on the processing equipment. Moreover, the conditions under which the neutron absorbers function must be considered. Since such artifacts, e.g. Raschig rings, are subjected to high amounts of degrading radiation and are often required to operate in acidic or other highly corrosive environments, the lifetime of most such fluorine substituted plastic or glass filled artifacts will be substantially reduced due to degrading of the glass or plastic and leaching of the filler. In addition to the problems caused by leaching of filler or plastic, many of the proposed plastics, such as polyvinylidine fluoride and polyvinyl chloride, absorb undue amounts of acid causing swelling and subsequent reduction in performance of the resulting product. Furthermore, in the case of glass Raschig rings, the relatively large amount of breakage, compounded by the leaching of filler material, results in a relatively short life for the ring. Subsequently, it is necessary to periodically dispose of large quantities of broken or otherwise unserviceable radioactive Raschig rings.
There is also a need in the art for materials suitable for use in pipes, blow molded or roto-molded vessels, etc. to be used for transporting or containing radioactive material. The materials required for such applications would have to exhibit the same properties as previously described, i.e. they would have to possess high neutron absorbing properties, be easily processable and not subject to degradation in radioactive or acidic enviornments. Moreover, although many applications deal with only thermal neutrons, i.e. neutrons which are of relatively low energy such as are given off as a consequence of natural radioactive decay, there are an increasing number of cases, such as in breeder reactors or nuclear weapons, where it may be desirable to eliminate certain atoms, particularly hydrogen, from the plastic polymer in order to prevent their participation in a nuclear reaction.
There is thus a need for easily processable, corrosion resistant effective nuclear shielding materials which can be highly filled without brittleness or loss in mechanical properties and which will maintain their superior properties even after exposure to intense radiation and/or highly acidic environments.